Field of the Invention
The invention relates to a method for monitoring secondary current circuits in an ignition vice of an external ignited internal combustion engine, having at least to primary current circuits each being closed or opened by means of one ignition end stage transistor. The invention also relates to an apparatus for performing the method.
Electric ignition devices for externally ignited internal combustion engines as a rule use one or more ignition coils as energy reservoirs for ignition sparks in order to ignite a fuel-air mixture in individual cylinders. During a certain closing time prior to the tripping of ignition, a primary winding of the associated ignition coil experiences a flow through it of an increasing current, and in the process stores energy in itself. Turning off the primary current trips an ignition, in that the stored energy flows through the secondary winding of the ignition coil into the spark plug, where it is broken down into the ignition spark.
If there is an interruption in the secondary current circuit of the ignition coil that cannot be bridged by the high voltage, then the energy stored in the ignition coil, after the opening of the primary current circuit in the ignition end stage transistor, is converted into heat once that transistor has been made conducting by means of its protective wiring, as a consequence of the excess voltage. Heretofore, monitoring the secondary current circuits in simple engine or ignition control devices has been too complicated and expensive and was therefore not done. In order to avoid overheating of the ignition end stage transistors, a heat dissipation line of the ignition end stage is made sufficiently oversized to avoid damage to the end stage in the event of a failure.
An interruption of the secondary current circuit cannot be detected by measuring the current or voltage on the primary side of the ignition coil, but it can be detected on the secondary side (from an overly low current or excessively high voltage). However, because of the high voltages involved, making that measurement is complicated and expensive.
Published European Application No. 0 470 277 A1 discloses an ignition device for internal combustion engines in which a sensor is disposed between the ignition coil and ground. The duration and amplitude of the secondary current can be measured by that sensor and evaluated in the microprocessor of the ignition control unit as to whether or not an undesired interruption in the secondary current circuit has occurred.